One use for ion implanters is in doping silicon wafers to form semi-conductor wafers suitable for further treatment during the process of fabricating integrated circuits. If the impurities used in doping the wafers can be ionized and shaped into the form of an ion beam, the ion beam can be used to dope the silicon wafers by causing the ion beam to strike the wafers in controlled concentrations.
One problem experienced in ion implantation doping of wafers is a problem of wafer charging. As the ion beam impacts the wafer, the wafer surface becomes positively-charged. The charging is often nonuniform and can create large electric fields at the wafer surface. These fields can damage the wafer, making it unsuitable for use as a semi-conductor material.
In some prior art implantation systems, an electron shower device is used to neutralize the space charge of the ion beam. Existing electron shower devices utilize secondary electron emissions caused when an energetic electron strikes a metal surface. Low-energy electrons from the metal surface are either trapped in the ion beam or are directed to impact the wafer surface thereby directly neutralizing the wafer.
The current density of electrons obtained by secondary emissions from a metal surface is limited by the potential difference between the ion beam and the metal electron-emitting surface. The ion beam potential drops as neutralization, takes place. The secondary emission electron current that can be extracted from the emitting surface will then decrease. In the case of a charged ion beam directed onto an electrically isolated wafer, the electron current must be equal to the ion beam current for the neutralizer to prevent wafer charging. If the beam potential is initially low, the wafer charges until the ion beam potential is large enough to extract a required amount of electron current from the electron-emitting surface.
A low potential ion beam can produce wafer charging. The center of the beam may remain positive and the exterior negative due to concentrations of negative charge surrounding the ion beam. In practice, it has been found that the space charge at the metallic electron emitting surface is partially neutralized by slow ions from residual gas ionization along the ion beam path. As a result, higher electron currents then would be expected from theory can be extracted and, in fact, prior art techniques take advantage of residual gas pressures to provide low-energy electrons.
U.S. Pat. No. 4,804,837 to Farley discloses a beam neutralizing system having a source of high-energy electrons. These electrons are deflected back and forth through a neutralizing region containing an ionizable gas. As the high-energy electrons pass through the region, they ionize the gas providing low-energy electrons for beam neutralization. The disclosure of the '837 patent to Farley is incorporated herein by reference. A stated goal of the '837 patent to Farley is to increase the time period electrons in the region of the ion beam encounter gas molecules thereby increasing the production of secondary electrons.
If an ion beam cannot be "sufficiently" neutralized, it tends to blow-up or expand due to the mutual repulsion of the positively-charged ions within the beam. To minimize the region of this blow-up, an electron barrier can be placed upstream from the electron shower. A typical ion implantation chamber has a support that rotates silicon wafers through the ion beam along a circular path so that the ion beam encounters a wafer, then a wafer support held at ground potential, and then a next subsequent wafer, etc. This causes the ion beam potential to rapidly fluctuate as the wafers pass through the ion beam. Variations in this beam potential are reduced by placement of a constant potential aperture plate upstream from a region of ion beam neutralization.
Fluctuation in beam potential is reduced by placement of a negatively biased aperture that produces a potential minimum along the ion beam. Electrons from the beam neutralizer cannot penetrate this aperture.
The use of suppression apertures has two undesirable consequences. The aperture introduces a boundary condition causing a sharp divergence in beam potential. This can exacerbate the beam blow-up downstream from the suppression aperture. Additionally, the electric field in the region of the aperture can deflect electrons from a beam neutralizer downstream to the region of the implantation chamber. This has the undesirable result of producing a region of positive charge at a central core of the ion beam and a negative charge at the outer periphery of the ion beam. Stated another way, the ion beam is neutral in a broad sense: but at the wafer surface, a positive charge build-up occurs at the wafer center and a negative charge occurs around the outer circumference. This results in the creation of large, undesirable electric fields.
U.S. Pat. No. 5,164,599 to Benveniste discloses an alternate beam neutralizer. A cylindrical electron source encircles an ion beam. Electrons emitted by an array of filaments spaced about the beam pass through the beam and collide with an inwardly facing wall of the neutralizer. Secondary electron emissions occur that neutralize the beam. The disclosure of the '599 patent to Benveniste is also incorporated by reference. The present invention presents an alternative to the systems disclosed in the Farley and Benveniste patents.